1. Field of the Invention
The present invention relates to a parallel robot.
2. Description of the Related Art
A parallel robot is provided, as a manipulator, with a so-called parallel mechanism in which a base section (or a stationary part) and a movable section (or an output part) are interconnected by a plurality of link structures arranged in parallel. In comparison with a serial robot such as an articulated robot, a parallel robot can easily ensure capabilities, such as high speed, high output, high accuracy, high stiffness, etc. In particular, in a parallel robot in which the movable section performs only a three-axis translational motion with respect to the base section, it is known that an auxiliary drive mechanism for controlling an orientation of a tool, such as a hand (also referred to as an end effector) mounted on the movable section, is provided in addition to a movable-section drive mechanism including the parallel link structures.
Japanese Examined Patent Publication (Kokoku) No. 4-45310 (JP4-45310B) describes a parallel robot provided, in addition to a movable-member drive mechanism (i.e., parallel link structures and link drive motors) having a parallel mechanism configuration and arranged between a base member and a movable member, with an auxiliary drive mechanism for rotating an operational member mounted on the movable member, which includes a motor disposed on the base member and a telescopic arm for transmitting the torque of the motor to the operational member. Japanese Unexamined Patent Publication (Kohyo) No. 2002-532269 (JP2002-532269A) also describes a parallel robot similar to that of JP4-45310B. An auxiliary drive mechanism in the parallel robot of JP2002-532269A is configured by an extensible shaft having a telescopic structure configured by assembling an outer tube and an inner shaft together. A “tubular hard bushing” is provided inside the outer tube, as a bearing member for linearly guiding the inner shaft inside the outer tube.
Japanese Patent No. 4109062 (JP4109062B) describes a parallel robot provided, in addition to a carrying-element drive mechanism (i.e., parallel link structures and link drive motors) having a parallel mechanism configuration and arranged between a base element and a carrying element, with an auxiliary drive mechanism for rotating a holding element mounted on the carrying element, which includes a motor disposed on the base element and a variable-length shaft for transmitting the torque of the motor to the holding element. In contrast to the telescopic configurations described in JP4-45310B and JP2002-532269A, the variable-length shaft includes a pair of bars adjacent to and parallely offset from each other, in which the bars are configured to move linearly with respect to each other while maintaining a mutually parallel state under the guiding function of “slide bearing elements” provided respectively on the bars. JP4109062B clearly describes a configuration wherein the bars of the variable-length shaft are coupled respectively to the motor on the base element and the holding element, through respective Cardan joints.
As described above, an auxiliary drive mechanism for controlling the orientation of a tool mounted on a movable section of a parallel robot conventionally uses an essentially extensible rod-shaped member having a telescopic configuration (JP4-45310B and JP2002-532269A) or a linearly-shiftable parallel configuration (JP4109062B), as a transmission member for transmitting the torque of a motor to the tool. Thus, the transmission member can elongate and contract while smoothly following the three-axis translational motion of the movable section obtained by the parallel mechanism, and thereby can transmit the torque of the motor to the tool on the movable section when the movable section is located at a desired (or commanded) spatial position within the operational space of the movable section.
In the above-described conventional parallel robot, the transmission member configured from the extensible rod-shaped member provided in the auxiliary drive mechanism for controlling a tool orientation is connected respectively to the motor on the base section and the tool on the movable section directly or indirectly through respective universal joints. Therefore, when the transmission member operates to elongate or contract, a linear bearing component arranged between a pair of rod-shaped elements in the telescopic configuration or the linearly-shiftable parallel configuration (the “tubular hard bushing” of JP2002-532269A or the “slide bearing elements” of JP4109062B) may interfere with a universal joint component provided at the distal end of each rod-shaped element, so as to restrict the elongation/contraction stroke of the transmission member, and thereby, an operational space inherently provided for the movable section may be constrained. Further, the transmission member configured by assembling a pair of rod-shaped elements tends to generate relatively-large inertial force, and thus there is concern that the inertial force of the transmission member may affect the motion of the movable section. If the diameter of each rod-shaped element is reduced to inhibit the inertial force, there is concern that the stiffness or operational accuracy of the transmission member may be degraded.